Hello. Was wondering if anyone could point me to absorption that might be considered bagged, bulk or loose, "chunk" type, rated for outdoor use. It would be used on top of a concrete pad, and under an AHU, for midband noise reduction. Dirt, grass and gravel have come to mind, but they want a concrete pad. Thanks.

Thanks. Was thinking of a crumbled or loose fill type absorption. It is on top of a concrete pad, directly under the chiller fans. There is already a masonry wall around it, so it is quite reflective inside the enclosure. Would not added absorption on a direct radiating surface offer some increase in noise reduction, mid band.

Easy enough to find out... drag a couple of bundles of fiberglass or rockwool semi-rigid boards over to the enclosure and spread em around inside to more or less line the floor and the interior side of the walls. Step back and take note of the difference. If it seems worth the effort come up with a simple way to fix the panels so as to cover the entirety of the interior surfaces [the bottom and the sides].

The standard approach would be to line the enclosure walls. I often specify 75mm mineral wool such as rockwool, minimum density 48 kg/m3, behind perforated metal, 25% minimum open area. Expanded metal (or expamet as it is known in UK) is cheaper than perforated steel panel. Be careful if placing any loose material below a chiller fan, as the fans typically extract air upwards and can shift several cubic meters per second, so anything loose could be sucked into the fans. If this is a commercial chiller, then it might be best to contact a specialist manufacturer in this area.

I've been involved on noise reduction for chillers in quite a few situations and have never come across placing material below it, as it will become soaked when it rains, catch dirt etc. Wall mounts or a direct attenuation of the fans is more common. Wall treatment has the added benefit that it removes a reflection path which can undermine the screening from the enclosure.

Can anyone elaborate on the principles for gun silencers for noise control. I assume it is a form of gas being slowed down or diverted and/or encased. I dont shoot, but cant figure out how they get 20-40 dB. Thanks

You can try https://www.acousticalfencing.com/product/acoustical-blankets/. It is sound blocking and sound sorption barriers that combine mass, flexibility, and limpness to block noise from transmitting from one area to another. They are available clear, non-reinforced, and reinforced in a variety of weights and styles to meet a multitude of applications.